CARBON PERGAMON Carbon40(2002)1217-1226 Short residence time graphitization of mesophase pitch-based carbon fib M.L. Greene.W. Schwartz,. J.W. Treleaven School of Materials Science and Engineering, Center for Advanced Engineering Fibers and Films, Clemson University Clemson. SC 29634.0907. USA BP Amoco Corporation, Alpharetta, GA 30202.3914, USA Abstract The effects of graphitization time and temperature on the properties of three mesophase pitch-based carbon fibers have been characterized Graphitization temperatures studied were 2400, 2700, and 3000C and residence times ranged from 0.7 to 3600 s. Helium pycnometry, measurements of fiber tow resistance, and X-ray diffraction were employed to study fiber properties. As anticipated, substantial variations in fiber properties were noted for the range of graphitization conditions studied and among the three fiber types. Significant structural evolution and property development occurred even at th shortest furnace residence times. For example, for one of the fibers, a furmace residence time of 0.7 s at 3000C resulted in a degree of graphitization value of -50%, a density of 1.98 g/cm, and an electrical resistivity of 6.3 un m(corresponding thermal conductivity -200 W mK ) A simple energy consumption analysis suggests that short residence time graphitization at high temperature may result in both lower costs and substantially higher production rates for fibers prepared from mesophase pitch. 2002 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibers, Mesophase pitch; B. Graphitization, Heat treatment; D. Electrical(electronic)properties 1. Introduction Two general approaches may be used to enhance the graphitization behavior of pitch-based precursors. First, the Carbon fibers are finding increasing use in a wide structural characteristics of the fiber precursor may be ariety of applications because of their superior strength controlled to promote more rapid structural development of and stiffness, low weight, and high thermal conductivity graphite-like fibers. Precursor structure may be controlled Applications range from sporting goods to aircraft struc- by the shape of the spinnerette die, drawdown ratio, tures and missile nose cones. The fibers are manufactured temperature, viscosity, flow conditions, and shear rate as from two main sources: polyacrylonitrile (PAN) and reported by Hamada [4], Matsumoto [5], and Edie [6]. The petroleum pitch (mesophase pitch) [1]- Carbon fibers second approach to optimize fiber properties involves prepared from pitch precursors typically utilize an ex control of graphitization time and temperature. Because of traction step to obtain the mesophase, followed by melt the high thermal input required for the development of pinning, and a thermal processing paradigm that includes graphitic structure, optimization of properties through this stabilization (thermosetting, 300C), carbonization method may also allow for a reduction in manufacturing (<1800C)[1, 2], and graphitization (up to 3000 C), costs, since graphitization represents a significant portion which promotes the development of the three-dimensional of the total production cost for graphite-like carbon fibers graphitic structure. a detailed review of the process has prepared from mesophase pitch. presented by Edie [3] In this study, the effects of graphitization time( from s1 s to I h) on the properties of mesophase pitch-based carbon orresponding author. Tel. + 1-864-656-7880; fax: +1-864- fibers have been characterized; results are reported for the 656-1453 degree of graphitization, electrical resistivity, and fiber E-mail address: bob. schwartz/@ces. clemson. edu (R W density. The purpose of these investigations was to evalu Schwartz) ate the rate at which graphitization occurred, and properties 0008-6223/02/S-see front matter 2002 Elsevier Science Ltd. All rights reserved PII:S0008-6223(01)00301-3
Carbon 40 (2002) 1217–1226 S hort residence time graphitization of mesophase pitch-based carbon fibers a a, b M.L. Greene , R.W. Schwartz , J.W. Treleaven * a School of Materials Science and Engineering, Center for Advanced Engineering Fibers and Films, Clemson University, Clemson, SC 29634-0907, USA b BP Amoco Corporation, Alpharetta, GA 30202-3914, USA Received 15 August 2000; accepted 3 October 2001 Abstract The effects of graphitization time and temperature on the properties of three mesophase pitch-based carbon fibers have been characterized. Graphitization temperatures studied were 2400, 2700, and 3000 8C and residence times ranged from 0.7 to 3600 s. Helium pycnometry, measurements of fiber tow resistance, and X-ray diffraction were employed to study fiber properties. As anticipated, substantial variations in fiber properties were noted for the range of graphitization conditions studied and among the three fiber types. Significant structural evolution and property development occurred even at the shortest furnace residence times. For example, for one of the fibers, a furnace residence time of 0.7 s at 3000 8C resulted in a 3 degree of graphitization value of |50%, a density of 1.98 g/cm , and an electrical resistivity of 6.3 mV m (corresponding 21 21 thermal conductivity |200 W m K ). A simple energy consumption analysis suggests that short residence time graphitization at high temperature may result in both lower costs and substantially higher production rates for fibers prepared from mesophase pitch. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers, Mesophase pitch; B. Graphitization, Heat treatment; D. Electrical (electronic) properties 1. Introduction Two general approaches may be used to enhance the graphitization behavior of pitch-based precursors. First, the Carbon fibers are finding increasing use in a wide structural characteristics of the fiber precursor may be variety of applications because of their superior strength controlled to promote more rapid structural development of and stiffness, low weight, and high thermal conductivity. graphite-like fibers. Precursor structure may be controlled Applications range from sporting goods to aircraft struc- by the shape of the spinnerette die, drawdown ratio, tures and missile nose cones. The fibers are manufactured temperature, viscosity, flow conditions, and shear rate as from two main sources: polyacrylonitrile (PAN) and reported by Hamada [4], Matsumoto [5], and Edie [6]. The petroleum pitch (mesophase pitch) [1]. Carbon fibers second approach to optimize fiber properties involves prepared from pitch precursors typically utilize an ex- control of graphitization time and temperature. Because of traction step to obtain the mesophase, followed by melt the high thermal input required for the development of spinning, and a thermal processing paradigm that includes graphitic structure, optimization of properties through this stabilization (thermosetting; |300 8C), carbonization method may also allow for a reduction in manufacturing (,1800 8C) [1,2], and graphitization (up to 3000 8C), costs, since graphitization represents a significant portion which promotes the development of the three-dimensional of the total production cost for graphite-like carbon fibers graphitic structure. A detailed review of the process has prepared from mesophase pitch. been presented by Edie [3]. In this study, the effects of graphitization time (from #1 s to 1 h) on the properties of mesophase pitch-based carbon fibers have been characterized; results are reported for the *Corresponding author. Tel.: 11-864-656-7880; fax: 11-864- 656-1453. degree of graphitization, electrical resistivity, and fiber E-mail address: bob.schwartz@ces.clemson.edu (R.W. density. The purpose of these investigations was to evaluSchwartz). ate the rate at which graphitization occurred, and properties 0008-6223/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(01)00301-3
l218 M L. Greene et al. Carbon 40(2002)1217-1226 ch as thermal conductivity were developed for three measure 18 inches longx9 inches wideX875 inches in different carbonized pitch precursors. Although the specific height. The heating element of the furnace and all internal nature of these precursors was not revealed by the supplier parts are graphite and the hot zone length was estimated at BP Amoco), we believe these results are of interest 5 inches. The temperature inside the hot zone was because there are few reports on the development of measured with an Ircon Mirage dual wavelength pyrometer carbon fiber properties at short graphitization times. Previ- [10, 11]. Argon was used as the purge gas usly, Richardson and Zehms [7 reported on heat treat Graphitization of the fibers was achieved by one of tw ment of pyrolytic carbon at temperatures between 2600 methods: continuous or batch processing. Continuous and 3000C for times ranging from 15 s to 10 min. processing was used to obtain hot zone residence times Time-dependent changes in gross dimensions and inter- rom 0.7 to 58.5 s, while batch processing was used to heat layer spacing were reported, although further analysis was the fibers for residence times ranging from 900 to 3600 s limited. Pandic [8] studied graphitization of a petroleum Graphitization temperatures of 2400, 2700 and 3000C coke and measured the interlayer spacing, stack height, and were evaluated coherence length for fibers processed at furmace residence For continuous mode operation, the fiber tow was times of 20 and 40 s, and 1, 3, and 4 min. The results threaded through the furnace from a supply reel at one end suggested that carbon passed through a rhombohedral form to a take-up winder at the opposite end using two pulleys during graphitization since the measured interlayer spacing at the entrance and exit of the furnace. Only sufficient was greater than the hexagonal form. Fischbach [9 tension was applied to the tows to eliminate any slack; it is erformed heat treatments on petroleum and coal tar pitch estimated that the pulling force was less than 50 g. Six cokes(similar to pitch mesophase) to study the kinetics of residence times: 0.7. 2.0.5.4. 12.5.. and 585s were graphitization for time periods lasting from 2 to 1000 min studied. These times were determined by measuring at temperatures of 2200 to 2900C Effective reaction rates time required for the fiber to travel through the hot zone increased by a factor of -10 over this temperature range and carrying out a rudimentary heat transfer calculation and the variation in the unit cell parameter was reported This analysis was performed using approximate heat hese earlier results indicate that graphitizable carbons transfer equations, assumptions regarding the thermal (such as mesophase) should transform rapidly at high conductivities of the fibers, modeling the fiber tow as a temperature. The results of the present study confirm that uniform infinite cylinder, and calculating the centerline significant graphitization may be attained within very short temperature. It was determined that the fiber tows attained times. We also show that fibers with comparatively higl the hot zone temperature nearly instantaneously [10]. The thermal conductivities may be produced using graphitiza cooling rate of the fibers heated by the continuous method tion times of <1 s. An energy analysis of the manufactur- was also very rapid; i.e., the fibers were essentially ing costs associated with the preparation of carbon fibers 'quenched from elevated temperature. Although the ef- under these conditions is presented, and we suggest that fects of cooling rate on fiber properties were not character- fiber production under these conditions appears econom- ized, the rapid cooling rate employed may also infuence cally favorable the structural properties of the fibers Batch mode processing was car wrapping approximately 8 m of the fiber tow around a 2. Experimen small piece of carbon felt which was then placed in the hot zone. All batch-mode samples were heated at a rate of-20 2. 1. Precursor fibers oC/min with an intermediate hold at 500C. The cooling rate of the furnace following the graphitization hold was The three pitch-based precursor fibers supplied by BP also estimated to be 20C/min. Heating and cooling rates Amoco(Alpharetta, GA, USA) were simply designated A, were obtained by using the manual control mode of the B, and C. The only information provided was that the furnace. Due to the high thermal activation energy of the fibers were carbonized, but not graphitized, implying that graphitization process [9, 12, time at temperatures beloy they had been heat treated to temperatures less than 1800 the final heat treatment temperature was neglected in the C. Fibers a and c were difficult to handle and would eat treatment analysis and was not included in the break if manipulated too harshly. Fiber B, in contrast, was isothermal residence times reported quite easy to handle without difficulty. Fiber diameter, shape, and microstructure were not characterized 2.3 Fiber characterisation 2.2. Fiber heat treatment in batch and continuous modes The densities of the precursor and heat treated fibers were measured by He pycnometry using a Micromeretics A water-cooled resistance furnace (Bethlehem Ad- AccuPyc 1330. To increase precision, the sample cell was vanced Materials, Knoxville, TN, USA) was used for fiber purged 10 times prior to each measurement and each heat treatment. The exterior dimensions of the furnace sample was run 25 times. The calculated standard devia-
1218 M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 such as thermal conductivity were developed for three measure 18 inches long39 inches wide38.75 inches in different carbonized pitch precursors. Although the specific height. The heating element of the furnace and all internal nature of these precursors was not revealed by the supplier parts are graphite and the hot zone length was estimated at (BP Amoco), we believe these results are of interest 3.5 inches. The temperature inside the hot zone was because there are few reports on the development of measured with an Ircon Mirage dual wavelength pyrometer carbon fiber properties at short graphitization times. Previ- [10,11]. Argon was used as the purge gas. ously, Richardson and Zehms [7] reported on heat treat- Graphitization of the fibers was achieved by one of two ment of pyrolytic carbon at temperatures between 2600 methods: continuous or batch processing. Continuous and 3000 8C for times ranging from 15 s to 10 min. processing was used to obtain hot zone residence times Time-dependent changes in gross dimensions and inter- from 0.7 to 58.5 s, while batch processing was used to heat layer spacing were reported, although further analysis was the fibers for residence times ranging from 900 to 3600 s. limited. Pandic [8] studied graphitization of a petroleum Graphitization temperatures of 2400, 2700 and 3000 8C coke and measured the interlayer spacing, stack height, and were evaluated. coherence length for fibers processed at furnace residence For continuous mode operation, the fiber tow was times of 20 and 40 s, and 1, 3, and 4 min. The results threaded through the furnace from a supply reel at one end suggested that carbon passed through a rhombohedral form to a take-up winder at the opposite end using two pulleys during graphitization since the measured interlayer spacing at the entrance and exit of the furnace. Only sufficient was greater than the hexagonal form. Fischbach [9] tension was applied to the tows to eliminate any slack; it is performed heat treatments on petroleum and coal tar pitch estimated that the pulling force was less than 50 g. Six cokes (similar to pitch mesophase) to study the kinetics of residence times: 0.7, 2.0, 5.4, 12.5, 33.1, and 58.5 s were graphitization for time periods lasting from 2 to 1000 min studied. These times were determined by measuring the at temperatures of 2200 to 2900 8C. Effective reaction rates time required for the fiber to travel through the hot zone 5 increased by a factor of |10 over this temperature range and carrying out a rudimentary heat transfer calculation. and the variation in the unit cell parameter was reported. This analysis was performed using approximate heat These earlier results indicate that graphitizable carbons transfer equations, assumptions regarding the thermal (such as mesophase) should transform rapidly at high conductivities of the fibers, modeling the fiber tow as a temperature. The results of the present study confirm that uniform infinite cylinder, and calculating the centerline significant graphitization may be attained within very short temperature. It was determined that the fiber tows attained times. We also show that fibers with comparatively high the hot zone temperature nearly instantaneously [10]. The thermal conductivities may be produced using graphitiza- cooling rate of the fibers heated by the continuous method tion times of ,1 s. An energy analysis of the manufactur- was also very rapid; i.e., the fibers were essentially ing costs associated with the preparation of carbon fibers ‘quenched’ from elevated temperature. Although the efunder these conditions is presented, and we suggest that fects of cooling rate on fiber properties were not character- fiber production under these conditions appears econom- ized, the rapid cooling rate employed may also influence ically favorable. the structural properties of the fibers. Batch mode processing was carried out by loosely wrapping approximately 8 m of the fiber tow around a 2. Experimental small piece of carbon felt which was then placed in the hot zone. All batch-mode samples were heated at a rate of |20 2 .1. Precursor fibers 8C/min with an intermediate hold at 500 8C. The cooling rate of the furnace following the graphitization hold was The three pitch-based precursor fibers supplied by BP also estimated to be 20 8C/min. Heating and cooling rates Amoco (Alpharetta, GA, USA) were simply designated A, were obtained by using the manual control mode of the B, and C. The only information provided was that the furnace. Due to the high thermal activation energy of the fibers were carbonized, but not graphitized, implying that graphitization process [9,12], time at temperatures below they had been heat treated to temperatures less than 1800 the final heat treatment temperature was neglected in the 8C. Fibers A and C were difficult to handle and would heat treatment analysis and was not included in the break if manipulated too harshly. Fiber B, in contrast, was isothermal residence times reported. quite easy to handle without difficulty. Fiber diameter, shape, and microstructure were not characterized. 2 .3. Fiber characterization 2 .2. Fiber heat treatment in batch and continuous modes The densities of the precursor and heat treated fibers were measured by He pycnometry using a Micromeretics A water-cooled resistance furnace (Bethlehem Ad- AccuPyc 1330. To increase precision, the sample cell was vanced Materials, Knoxville, TN, USA) was used for fiber purged 10 times prior to each measurement and each heat treatment. The exterior dimensions of the furnace sample was run 25 times. The calculated standard devia-
.L. Greene et al. Carbon 40(2002)1217-1226 tions typically ranged from 0.002 g/cm, although determination of dooz, the degree of graphitization was a few samples showed lower cm)and higher calculated according to (up to 0.008 g/cm) standard Density values 3.44-dooz were used both as an indication of the degree of graphene 8=.44-3354x100 plane alignment and for calculation of resistivity The resistivities of the fiber tows were determined with In this equation, 3.44 represents the interlayer spacing of measurement, tows were twisted approximately 50 times spacing for single crystal graphite(in A, e interlayer a micro-ohm meter. To improve the reproducibility of the turbostratic graphite (in A)and 3. 354 is the interlayer prior to measurement to contain any loose or broken filaments within the tow. The apparatus employed was constructed in-house and allowed for the characterization 3. Results and discussion of multiple 0.25 m segments of I m long fiber tows. To minimize instrumental resistance effects, large diameter 3. 1. Precursor fiber properties copper wire was used for connection to the micro-ohm meter and polished copper rods were employed to make The density, electrical resistivity, and interlayer spacing contact to the fiber tows. Resistivity, r(uf m), was of the precursor fibers were measured with the techniques calculated using described above and are reported in Table 1. All of the results suggest that Fiber Bb is the most turbostratic, i.e (R*)(p) tructurally, this fiber bears the least resemblance to where R is the measured resistance in n, Y is the yield of density and a much greater resistivity. The don spacing the fiber tow(g/m), Pr is the fiber density (Mg/m), and Is peak intensities, and peak widths of the three precursors is the segment length(m). Each s\ egar, reliable also suggest that Fiber B is more turbostratic in nature.In measured between eight and 10 times to obtain contrast, Fibers a and c are much more similar, with Fiber to 14 unl m and standard deviations were between 0.015 a demonstrating a structure that is more graphite-like and 0.066 Hn2 m. The thermal conductivities of the fibers were estimated 3.2 Fiber A alues using the lavin equation [13], a form of which is shown below: The effects of graphitization time density are shown in Fig. la. Although Fiber A has the K=4400/(+2.58)-295 (2) highest initial density, 1.8550 g/cm, significant addition densification occurs. even for short residence times. For a where K is the thermal conductivity (wmK) and r 0.7 s residence time, fiber density increased to 1.9045 the electrical resistivity (un m) as determined by Eq. (1). 1.9653, and 1.9823 g/cm for graphitization at 2400, 2700, Structural characterization of the fibers was achieved and 3000C, respectively. From a structural perspective with X-ray diffraction. Short lengths of tow were dipped (Eq(3)), the transition from a turbostratic to a graphiti into melted paraffin wax, and after solidifying, a 2.54 cm structure can only account for a density increase of -2.5% length of this composite was cut and affixed to an Therefore, void, or pore, elimination within the fiber must aluminum ring. This was then set on top of a sapphire disc also contribute to densification. in the sample cup of a Scintag XDs 2000 diffractometer For longer residence times, densification continues (Cu Ka radiation) and the diffraction pattern between 20 all heat treatment temperatures. For example, after 58.5 s, and 90 20 was obtained at a scan rate of 0.50 per minute tensity values ranged from 1.9512(2400C)to 2.0049 The scans were analyzed for any anomalous shifts in peak g/cm(2700C). While these values are lower than those position( due to imperfect calibration of the diffractometer) of graphite(2.1-2.3 g/cm), they indicate that substantial using the known peak positions of sapphire. Following densification occurs under these heat treatment conditions Table I Densities, resistivities, calculated thermal conductivities, and crystallographic properties of precursor fibers Fiber type Density Resistivity Thermal doozy peak position intensity(A U ) and FWHM(°26) 3.432/38,000/0.6 1.6793 Undetermined 3.454/100/3.4 1.8212 3.447/30,700/1.0 FWHM: peak full width at half maximum
M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 1219 3 tions typically ranged from 0.002 to 0.005 g/cm , although determination of d , the degree of graphitization was 002 3 a few samples showed lower (0.0015 g/cm ) and higher calculated according to 3 (up to 0.008 g/cm ) standard deviations. Density values 3.44 2 d002 were used both as an indication of the degree of graphene g 5 3 ]]]] 100 (3) 3.44 2 3.354 plane alignment and for calculation of resistivity. The resistivities of the fiber tows were determined with In this equation, 3.44 represents the interlayer spacing of ˚ a micro-ohm meter. To improve the reproducibility of the turbostratic graphite (in A) and 3.354 is the interlayer ˚ measurement, tows were twisted approximately 50 times spacing for single crystal graphite (in A). prior to measurement to contain any loose or broken filaments within the tow. The apparatus employed was constructed in-house and allowed for the characterization 3. Results and discussion of multiple 0.25 m segments of 1 m long fiber tows. To minimize instrumental resistance effects, large diameter 3 .1. Precursor fiber properties copper wire was used for connection to the micro-ohm meter and polished copper rods were employed to make The density, electrical resistivity, and interlayer spacing contact to the fiber tows. Resistivity, r (mV m), was of the precursor fibers were measured with the techniques calculated using described above and are reported in Table 1. All of the results suggest that Fiber B is the most turbostratic; i.e., r 5 (R*Y)/(r *l ) (1) f s structurally, this fiber bears the least resemblance to graphite-like carbon. This fiber has a substantially lower where R is the measured resistance in V, Y is the yield of 3 density and a much greater resistivity. The d spacings, 002 the fiber tow (g/m), r is the fiber density (Mg/m ), and l f s peak intensities, and peak widths of the three precursors is the segment length (m). Each fiber segment was also suggest that Fiber B is more turbostratic in nature. In measured between eight and 10 times to obtain a reliable contrast, Fibers A and C are much more similar, with Fiber average of the resistance. Resistivity values ranged from 2 A demonstrating a structure that is more graphite-like. to 14 mV m and standard deviations were between 0.015 and 0.066 mV m. 3 .2. Fiber A The thermal conductivities of the fibers were estimated from the measured resistivity values using the Lavin The effects of graphitization time and temperature on equation [13], a form of which is shown below: density are shown in Fig. 1a. Although Fiber A has the 3 highest initial density, 1.8550 g/cm , significant additional k 5 4400/(r 1 2.58) 2 295 (2) densification occurs, even for short residence times. For a 21 21 where k is the thermal conductivity (W m K ) and r is 0.7 s residence time, fiber density increased to 1.9045, 3 the electrical resistivity (mV m) as determined by Eq. (1). 1.9653, and 1.9823 g/cm for graphitization at 2400, 2700, Structural characterization of the fibers was achieved and 3000 8C, respectively. From a structural perspective with X-ray diffraction. Short lengths of tow were dipped (Eq. (3)), the transition from a turbostratic to a graphitic into melted paraffin wax, and after solidifying, a 2.54 cm structure can only account for a density increase of |2.5%. length of this composite was cut and affixed to an Therefore, void, or pore, elimination within the fiber must aluminum ring. This was then set on top of a sapphire disc also contribute to densification. in the sample cup of a Scintag XDS 2000 diffractometer For longer residence times, densification continues for (Cu Ka radiation) and the diffraction pattern between 20 all heat treatment temperatures. For example, after 58.5 s, and 908 2u was obtained at a scan rate of 0.508 per minute. density values ranged from 1.9512 (2400 8C) to 2.0049 3 The scans were analyzed for any anomalous shifts in peak g/cm (2700 8C). While these values are lower than those 3 position (due to imperfect calibration of the diffractometer) of graphite (2.1–2.3 g/cm ), they indicate that substantial using the known peak positions of sapphire. Following densification occurs under these heat treatment conditions. Table 1 Densities, resistivities, calculated thermal conductivities, and crystallographic properties of precursor fibers Fiber type Density Resistivity Thermal d(002) Peak position 23 ˚ (as-received) (g cm ) (mV m) conductivity (A); intensity (A.U.); 21 21 a (W m K ) and FWHM (82u ) A 1.8550 8.36 107 3.432/38,000/0.6 B 1.6793 14.19 Undetermined 3.454/100/3.4 C 1.8212 8.55 100 3.447/30,700/1.0 a FWHM: peak full width at half maximum
1220 M L. Greene et al. / Carbon 40 (2002)1217-1226 HTT=2700 2.0000 -0HTT=2400 1.9800 7.00 6.00 8 819400 300 1.9000 HTT=3000 口HT=2700 -HTT=2400 18800 ag Residence Time(seconds) 0010000 Log Residence Time(seconds) 0.6 -HTT=2400 0.1 10100100010000 Fig. 1.(a) Density, (b)resistivity /thermal conductivity, and(c)calculated degree of graphitization of Fiber A as a function of residence time at graphitization temperatures of 2400, 2700, and 3000C. Asterisks(*)indicate heat treatment times during which densification proceeds at Fig. 1a also suggests that the rate of densification is not not constant, as discussed previously [12]. Time periods at garithmically dependent on the graphitization time(nor is which the densification rate lags are indicated by asterisks. early dependent on time). Because the error associate but at this time. the reasons for the decreased densification with the measured density values is approximately two to rates at these times are not clear three times the symbol size, it appears that a simple Fig. 1b illustrates the effects of graphitization time and straight line fit does not adequately describe the densifica- temperature on the electrical resistivity of the fiber. The tion behavior. This implies that the rate of densification is variation in resistivity as a function of time is greatest at
1220 M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 Fig. 1. (a) Density, (b) resistivity/thermal conductivity, and (c) calculated degree of graphitization of Fiber A as a function of residence time at graphitization temperatures of 2400, 2700, and 3000 8C. Asterisks (*) indicate heat treatment times during which densification proceeds at a slower rate. Fig. 1a also suggests that the rate of densification is not not constant, as discussed previously [12]. Time periods at logarithmically dependent on the graphitization time (nor is which the densification rate lags are indicated by asterisks, it linearly dependent on time). Because the error associated but at this time, the reasons for the decreased densification with the measured density values is approximately two to rates at these times are not clear. three times the symbol size, it appears that a simple Fig. 1b illustrates the effects of graphitization time and straight line fit does not adequately describe the densifica- temperature on the electrical resistivity of the fiber. The tion behavior. This implies that the rate of densification is variation in resistivity as a function of time is greatest at
M L. Greene et al. Carbon 40(2002)1217-1226 3000C, but a significant decrease in resistivity is noted with the 2700C heat treatment actually yielding an for all temperatures. At 2700C, the resistivity of the apparently higher degree of graphitization. While some carbonized precursor is reduced by 20% after heat treat- degradation in the structure at 3000C might give rise to ment for 5.4 s, by more than 45% for a residence time of this effect, it seems equally likely that there is a slight error approximately I min, and by more than 60% for a 30 min in the measurement of the dooz spacing which results in an heat treatment. Since the electrical resistivity of these error in the degree of graphitization calculated by eq. (3) materials is dependent on structural perfection, these This equation is sensitive to small changes in d-spacing esults indicate that significant order develops within Fiber and a relatively minor measurement error of 0.001 A in A within the first minute or two of graphitization, both at doo can give an error in the calculated percent graphite of 2700 and 3000C. As for density, the resistivity behavior more than 1% of the fiber heated at 2400oC was different than the fibers Finally, we consider the calculated thermal conductivity heated at 2700 and 3000 C. Also, as observed for of Fiber A as a function of graphitization time and densification, the change in electrical resistivity as a temperature. These results, shown in Fig. Ib, were ob- function of heat treatment time is non-uniform; for longer tained by the method of Lavin [13] for the conversion of heat treatment times at 2700 and 3000C, the rate of electrical resistivity. Although the identity of this fiber is hange of the resistivity becomes small not known, the initial thermal conductivity of 107 Wm The structural analysis(Eq (3) of the fibers graphitized K(Table 1)indicates it is the best thermally conducting under different conditions is presented in Fig. Ic. Again, precursor of the three evaluated. A thermal conductivity there is a large difference in the degree of graphitizat scale, obtained using Eq(2), is marked on the right side of etween the fibers heat treated at 2400C and those at the figure. At 3000C, even for a residence time as short 2700 and 3000C, in agreement with expectations from as 0.7 s, the thermal conductivity increases to <200 W density and electrical resistivity. Even for the longest heat mK; for heat treatments as short at 12.5 s, a thermal treatment times, at 2400C, the graphitization of the fibers conductivity of 374 wmK is obtained. These values is below 25%. This suggests that significant densification, compare favorably with those of commercially available as well as enhancements in electrical and thermal con- P-75 and P-100 [13] and approach the thermal conductivity ductivity, can be obtained without the extensive formation of copper. With longer heat treatment times(1800 S), of graphitic structure in this pitch-based fiber thermal conductivity of nearly 700 w mKwas In contrast to the results observed at 2400C, significa levelopment of graphitic structure occurs at 2700 and 000 C, even for short heat treatment times. For a residence time of 0.7 s, at 2700C, a g value of 23% was 3.3. Fiber B obtained, while at 3000C, a degree of graphitization of 51% was determined. This indicates that graphene plane As-received Fiber B had a significantly lower density alignment can occur rapidly in these materials at the (1.6793 g/cm)and higher resistivity (14.19 un m)than elevated temperatures employed for graphitization and Fiber A. On this basis, we speculate that the fiber has a onfirms expectations from earlier studies on cokes [91 ower mesophase content than Fiber A, or that it possesses The results for degree of graphitization are well correlated less structural order. The measured density of Fiber b as a with the relatively high densities that were observed fo function of graphitization time and temperature is shown in these heat treatment conditions. Fig. Ic also confirms the Fig. 2a. While the general relationships between density suggestion of the density and resistivity results that the rate and graphitization time and temperature are similar to of graphitization of this fiber is non-uniform in time and those of Fiber A, there are notable differences. For Fiber that the behavior at 2700 and 3000C is more similar than B, graphitization at 2400 and 2700C yielded relatively at 2400 C. The curves at 2700 and 3000C show an minor differences in fiber density, with substantially more apparent step in the transformation process beyond which densification occurring at 3000C. Another difference the rate of graphitization significantly decreases. While a between these fibers is the final densities that were ontributing factor to this behavior may be the two obtained, irrespective of heat treatment time. Comparing ifferent graphitization methods used(continuous and the results for the continuously processed fibers, the final batch), analogous steps have previously been seen in X-ray density(1.8536 g/cm)of Fiber B subjected to the most diffraction and diamagnetic susceptibility studies per- vere graphitization conditions(3000C, 58.5 s) was still formed by Pacault and coworkers [12] less than the initial density of Fiber A. However, the The graphitization results for the samples prepared in relative increase in density as a function of residence time the batch mode at long residence are not as consistent as as greater for B than A. As with Fiber A, Fiber b also those prepared by continuous processing. A significant exhibited certain temperature regimes where the rate of increase in graphite content is observed when the heat densification decreased. We also note that the densification treatment temperature is raised from 2400 to 2700C, but behavior of Fiber B is, in general, somewhat less depen- the results obtained at 2700 and 3000C are very similar, dent on graphitization time and temperature than Fiber A
M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 1221 3000 8C, but a significant decrease in resistivity is noted with the 2700 8C heat treatment actually yielding an for all temperatures. At 2700 8C, the resistivity of the apparently higher degree of graphitization. While some carbonized precursor is reduced by 20% after heat treat- degradation in the structure at 3000 8C might give rise to ment for 5.4 s, by more than 45% for a residence time of this effect, it seems equally likely that there is a slight error approximately 1 min, and by more than 60% for a 30 min in the measurement of the d spacing which results in an 002 heat treatment. Since the electrical resistivity of these error in the degree of graphitization calculated by Eq. (3). materials is dependent on structural perfection, these This equation is sensitive to small changes in d-spacing ˚ results indicate that significant order develops within Fiber and a relatively minor measurement error of 0.001 A in A within the first minute or two of graphitization, both at d can give an error in the calculated percent graphite of 002 2700 and 3000 8C. As for density, the resistivity behavior more than 1%. of the fiber heated at 2400 8C was different than the fibers Finally, we consider the calculated thermal conductivity heated at 2700 and 3000 8C. Also, as observed for of Fiber A as a function of graphitization time and densification, the change in electrical resistivity as a temperature. These results, shown in Fig. 1b, were obfunction of heat treatment time is non-uniform; for longer tained by the method of Lavin [13] for the conversion of heat treatment times at 2700 and 3000 8C, the rate of electrical resistivity. Although the identity of this fiber is 21 change of the resistivity becomes smaller. not known, the initial thermal conductivity of 107 W m 21 The structural analysis (Eq. (3)) of the fibers graphitized K (Table 1) indicates it is the best thermally conducting under different conditions is presented in Fig. 1c. Again, precursor of the three evaluated. A thermal conductivity there is a large difference in the degree of graphitization scale, obtained using Eq. (2), is marked on the right side of between the fibers heat treated at 2400 8C and those at the figure. At 3000 8C, even for a residence time as short 2700 and 3000 8C, in agreement with expectations from as 0.7 s, the thermal conductivity increases to |200 W 21 21 density and electrical resistivity. Even for the longest heat m K ; for heat treatments as short at 12.5 s, a thermal 21 21 treatment times, at 2400 8C, the graphitization of the fibers conductivity of 374 W m K is obtained. These values is below 25%. This suggests that significant densification, compare favorably with those of commercially available as well as enhancements in electrical and thermal con- P-75 and P-100 [13] and approach the thermal conductivity ductivity, can be obtained without the extensive formation of copper. With longer heat treatment times (1800 s), a 21 21 of graphitic structure in this pitch-based fiber. thermal conductivity of nearly 700 W m K was In contrast to the results observed at 2400 8C, significant obtained. development of graphitic structure occurs at 2700 and 3000 8C, even for short heat treatment times. For a residence time of 0.7 s, at 2700 8C, a g value of 23% was 3 .3. Fiber B obtained, while at 3000 8C, a degree of graphitization of 51% was determined. This indicates that graphene plane As-received Fiber B had a significantly lower density 3 alignment can occur rapidly in these materials at the (1.6793 g/cm ) and higher resistivity (14.19 mV m) than elevated temperatures employed for graphitization and Fiber A. On this basis, we speculate that the fiber has a confirms expectations from earlier studies on cokes [9]. lower mesophase content than Fiber A, or that it possesses The results for degree of graphitization are well correlated less structural order. The measured density of Fiber B as a with the relatively high densities that were observed for function of graphitization time and temperature is shown in these heat treatment conditions. Fig. 1c also confirms the Fig. 2a. While the general relationships between density suggestion of the density and resistivity results that the rate and graphitization time and temperature are similar to of graphitization of this fiber is non-uniform in time and those of Fiber A, there are notable differences. For Fiber that the behavior at 2700 and 3000 8C is more similar than B, graphitization at 2400 and 2700 8C yielded relatively at 2400 8C. The curves at 2700 and 3000 8C show an minor differences in fiber density, with substantially more apparent step in the transformation process beyond which densification occurring at 3000 8C. Another difference the rate of graphitization significantly decreases. While a between these fibers is the final densities that were contributing factor to this behavior may be the two obtained, irrespective of heat treatment time. Comparing different graphitization methods used (continuous and the results for the continuously processed fibers, the final 3 batch), analogous steps have previously been seen in X-ray density (1.8536 g/cm ) of Fiber B subjected to the most diffraction and diamagnetic susceptibility studies per- severe graphitization conditions (3000 8C, 58.5 s) was still formed by Pacault and coworkers [12]. less than the initial density of Fiber A. However, the The graphitization results for the samples prepared in relative increase in density as a function of residence time the batch mode at long residence are not as consistent as was greater for B than A. As with Fiber A, Fiber B also those prepared by continuous processing. A significant exhibited certain temperature regimes where the rate of increase in graphite content is observed when the heat densification decreased. We also note that the densification treatment temperature is raised from 2400 to 2700 8C, but behavior of Fiber B is, in general, somewhat less depenthe results obtained at 2700 and 3000 8C are very similar, dent on graphitization time and temperature than Fiber A
M L. Greene et al. / Carbon 40 (2002)1217-1226 1.8600 1200 HTT=3000 .8500-HTT=2700 -HT=2400 1000 1.8400 1.8300 1.8200 218 1.8100 1.8000 4.00 200-HT=3000 1.7800 HTT=2700 C-HTT=2400 1.7700 000 100 Log Residence Time(seconds) (b) 06 : △HTT Fig. 2.(a) Density, (b)resistivity/thermal conductivity, and(c)calculated degree of graphitization of Fiber B as a function of residence time at graphitization temperatures of 2400, 2700, and 3000C. Asterisks(*)indicate heat treatment times during which densification proceeds at a slower rate Fig. 2b shows the electrical resistivity of Fiber B as a 14 19 uQ m, and the broadest door diffraction peak, function of graphitization conditions. Fiber B(as-received) suggesting a low degree of structural order in this fiber had the highest electrical resistivity of the three precursors, With graphitization, the resistivity decreased significantly
1222 M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 Fig. 2. (a) Density, (b) resistivity/thermal conductivity, and (c) calculated degree of graphitization of Fiber B as a function of residence time at graphitization temperatures of 2400, 2700, and 3000 8C. Asterisks (*) indicate heat treatment times during which densification proceeds at a slower rate. Fig. 2b shows the electrical resistivity of Fiber B as a 14.19 mV m, and the broadest d diffraction peak, 002 function of graphitization conditions. Fiber B (as-received) suggesting a low degree of structural order in this fiber. had the highest electrical resistivity of the three precursors, With graphitization, the resistivity decreased significantly
M L. Greene et al. Carbon 40(2002)1217-1226 but remained much higher than Fiber A for analogous heat the smaller data set for this fiber, and the fact that the continuously processed Fiber A samples treated at 2400C The estimated thermal conductivity values of Fiber b were not studied by XRD. However, it appears there are ohitized under different time and temperature condi- significant differences in the extent of graphitization of the tions are indicated by the scale on the right side of Fig. 2b two fibers when heated at 2700C. For analogous heat and ranged from 55 WmK(0.7 s, 2400C)to -450 treatment times at 2700C, Fiber C demonstrates a much Wm K(3600 s, 3000oC). Values between 90 and 330 lower g value than Fiber A, while at the same time, the WmK may be obtained for a residence time under 1 Fiber C samples display similar electrical resistivities. This min depending upon graphitization temperature mplies that the degree of graphitization is not the sole The g values for Fiber B calculated from Eq.(3)are factor in defining the observed electrical(and, hence presented in Fig. 2c and, as expected, were much lower thermal) properties of these materials, as expected. Other than for Fiber A, for equivalent heat treatment conditions factors, such as defect content, layer orientation, coherence However, the general trends of increased degree of length, etc also play a role graphitization with longer residence time, or higher heat treatment temperature, were again observed. At 3000C, graphitization appears to reach a plateau, yielding ag 3.5. Correlation of graphitization behavior to physical value <0.50 after going through a rapid increase for times properties between 12.5 and 33.1 s Finally, comparing the graphitization results(Figs. lc The densification and electrical resistivity responses as a and 2c) with the resistivity values(Figs. Ib and 2b), it function of degree of graphitization are shown in Figs. 4 appears that a measurable degree of graphitization of the and 5, respectively. All three fibers show a trend of fiber(approximately 10% or greater) is required to attain a increasing density with an increase in graphite content; resistivity below 8 unl m and a thermal conductivity above however, the responses are non-linear and appear to show 120 Wm K. For example, Fiber A heat treated at teps, similar to those noted for densification vs. graphiti 2700C for 0.7 s displays a resistivity of 7.08 un m with zation time. The results for Fiber B are confined to a corresponding graphitization degree of 23.3%. Fiber b comparatively low degrees of graphitization and low heat treated at 2700 C for 33 1 s demonstrates an density values. In contrast, Fibers a and C show that a electrical resistivity of 7.29 un cm and a degree of wide range of g values can contribute to comparatively graphitization of 10.5%. In contrast, all fibers heat treated small changes in fiber density. This is to be expected since under conditions that yielded resistivity values greater than the structural changes(variations in the dooz spacing)that 8.0 Hn m had g values of a few percent or less. The occur with graphitization between the purely turbostratic relationship between electrical resistivity and graphitize- and graphitic structures can only account for a density tion is discussed further below. In general, it is observed increase of -2.5%. Because the overall increase in density that there is a strong correlation between properties such as resulting from graphitization ranges from 8.1 to 10.4% density and electrical resistivity and the degree of graphiti- other processes, such as void collapse and an increase in zation of the fiber; however, residual effects of precursor layer orientation, neither of which affects g, must also play type on this relationship persist after heat treatment. a major role in densification. Thus, the apparent correlation between density and graphitization is largely fortuitous because these other densification processes proceed simul 3. 4. Fiber C taneously with graphitization. The relationships between the electrical resistivities and The density and electrical resistivity behaviors of Fiber degrees of graphitization of the three fibers(Fig. 5)are C as a function of graphitization are similar to Fiber A, as also informative. For Fibers B and C, the relationship shown in Fig. 3. The density of this precursor fiber, 1.8212 between these two properties is nearly linear, and several g/cm, was intermediate to Fibers A and B, as were the of the data points overlap. Fiber A also shows a strong densities obtained for analogous graphitization conditions. correlation, as would be expected due to the importance of The resistivity of as-received Fiber C,8.55 un2 m, was structure on resistivity. However, this fiber demonstrates a also intermediate between Fibers A and B. Fig. 3b shows lower dependence of resistivity on degree of graphitize that the general characteristics of the resistivity vs heat tion. A more in-depth understanding of these relationships treatment response are very similar to Fiber A, and that the requires further characterization of additional fiber struc thermal conductivities of these fibers ranged from -100 tural properties such as layer orientation and coherence (0.7s,2400℃)to~650Wm-1K-(1800s,3000°C length. Unfortunately, the characterization of these prop The degree of graphitization for Fiber C samples erties has not yet been completed. The results do indicate, processed continuously at 2400 and 2700C was measured though, that expected relationships between fiber prop- and the results are presented in Fig 3c. It is difficult to erties and degree of graphitization exist, and that thes directly compare these results with those of Fiber A, due to relationships are dependent on precursor type
M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 1223 but remained much higher than Fiber A for analogous heat the smaller data set for this fiber, and the fact that the treatment conditions. continuously processed Fiber A samples treated at 2400 8C The estimated thermal conductivity values of Fiber B were not studied by XRD. However, it appears there are graphitized under different time and temperature condi- significant differences in the extent of graphitization of the tions are indicated by the scale on the right side of Fig. 2b two fibers when heated at 2700 8C. For analogous heat 21 21 and ranged from 55 W m K (0.7 s, 2400 8C) to |450 treatment times at 2700 8C, Fiber C demonstrates a much 21 21 W m K (3600 s, 3000 8C).Values between 90 and 330 lower g value than Fiber A, while at the same time, the 21 21 W m K may be obtained for a residence time under 1 Fiber C samples display similar electrical resistivities. This min depending upon graphitization temperature. implies that the degree of graphitization is not the sole The g values for Fiber B calculated from Eq. (3) are factor in defining the observed electrical (and, hence, presented in Fig. 2c and, as expected, were much lower thermal) properties of these materials, as expected. Other than for Fiber A, for equivalent heat treatment conditions. factors, such as defect content, layer orientation, coherence However, the general trends of increased degree of length, etc., also play a role. graphitization with longer residence time, or higher heat treatment temperature, were again observed. At 3000 8C, graphitization appears to reach a plateau, yielding a g 3 .5. Correlation of graphitization behavior to physical value ,0.50 after going through a rapid increase for times properties between 12.5 and 33.1 s. Finally, comparing the graphitization results (Figs. 1c The densification and electrical resistivity responses as a and 2c) with the resistivity values (Figs. 1b and 2b), it function of degree of graphitization are shown in Figs. 4 appears that a measurable degree of graphitization of the and 5, respectively. All three fibers show a trend of fiber (approximately 10% or greater) is required to attain a increasing density with an increase in graphite content; resistivity below 8 mV m and a thermal conductivity above however, the responses are non-linear and appear to show 21 21 120 W m K . For example, Fiber A heat treated at steps, similar to those noted for densification vs. graphiti- 2700 8C for 0.7 s displays a resistivity of 7.08 mV m with zation time. The results for Fiber B are confined to a corresponding graphitization degree of 23.3%. Fiber B comparatively low degrees of graphitization and low heat treated at 2700 8C for 33.1 s demonstrates an density values. In contrast, Fibers A and C show that a electrical resistivity of 7.29 mV cm and a degree of wide range of g values can contribute to comparatively graphitization of 10.5%. In contrast, all fibers heat treated small changes in fiber density. This is to be expected since under conditions that yielded resistivity values greater than the structural changes (variations in the d spacing) that 002 8.0 mV m had g values of a few percent or less. The occur with graphitization between the purely turbostratic relationship between electrical resistivity and graphitiza- and graphitic structures can only account for a density tion is discussed further below. In general, it is observed increase of |2.5%. Because the overall increase in density that there is a strong correlation between properties such as resulting from graphitization ranges from 8.1 to 10.4%, density and electrical resistivity and the degree of graphiti- other processes, such as void collapse and an increase in zation of the fiber; however, residual effects of precursor layer orientation, neither of which affects g, must also play type on this relationship persist after heat treatment. a major role in densification. Thus, the apparent correlation between density and graphitization is largely fortuitous because these other densification processes proceed simul- 3 .4. Fiber C taneously with graphitization. The relationships between the electrical resistivities and The density and electrical resistivity behaviors of Fiber degrees of graphitization of the three fibers (Fig. 5) are C as a function of graphitization are similar to Fiber A, as also informative. For Fibers B and C, the relationship shown in Fig. 3. The density of this precursor fiber, 1.8212 between these two properties is nearly linear, and several 3 g/cm , was intermediate to Fibers A and B, as were the of the data points overlap. Fiber A also shows a strong densities obtained for analogous graphitization conditions. correlation, as would be expected due to the importance of The resistivity of as-received Fiber C, 8.55 mV m, was structure on resistivity. However, this fiber demonstrates a also intermediate between Fibers A and B. Fig. 3b shows lower dependence of resistivity on degree of graphitizathat the general characteristics of the resistivity vs. heat tion. A more in-depth understanding of these relationships treatment response are very similar to Fiber A, and that the requires further characterization of additional fiber structhermal conductivities of these fibers ranged from |100 tural properties such as layer orientation and coherence 21 21 (0.7 s, 2400 8C) to |650 W m K (1800 s, 3000 8C). length. Unfortunately, the characterization of these propThe degree of graphitization for Fiber C samples erties has not yet been completed. The results do indicate, processed continuously at 2400 and 2700 8C was measured though, that expected relationships between fiber propand the results are presented in Fig. 3c. It is difficult to erties and degree of graphitization exist, and that these directly compare these results with those of Fiber A, due to relationships are dependent on precursor type
1224 M L. Greene et al. / Carbon 40 (2002)1217-1226 20000 8.00 7.00 8 55.00 19400 4.00 374 19200 19000 2.00 HTT=3000 HTT=3000 一HTT=2700 100 口HTT=2700 -HTT=2400 000 1001000100 Log Residence Time(seconds) (a) HTT=2700 - -HT 02 0.1 0.05 Log Residence Time( seconds) Fig 3.(a) Density, (b)resistivity/thermal conductivity, and (c)calculated degree of graphitization of Fiber C as a function of residence time at graphitization temperatures of 2400, 2700, and 3000C. Asterisks(*)indicate heat treatment times during which densification proceeds at a slower rate 3.6. Energy analysis of fiber and temperature conditions that resulted in fibers with similar electrical resistivities/thermal conductivities were To evaluate the graphitization conditions that were most first identified. Then, utility costs in kwh to produce the energy efficient for the production of carbon fibers, time fibers under these different conditions were calculated
1224 M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 Fig. 3. (a) Density, (b) resistivity/thermal conductivity, and (c) calculated degree of graphitization of Fiber C as a function of residence time at graphitization temperatures of 2400, 2700, and 3000 8C. Asterisks (*) indicate heat treatment times during which densification proceeds at a slower rate. 3 .6. Energy analysis of fiber production and temperature conditions that resulted in fibers with similar electrical resistivities/thermal conductivities were To evaluate the graphitization conditions that were most first identified. Then, utility costs in kWh to produce the energy efficient for the production of carbon fibers, time fibers under these different conditions were calculated
M L. Greene et al. / Carbon 40 (2002)1217-1226 05 different residence times reported imply that different production rates can be expected for the manufacture of carbon fibers with similar properties. For example, by graphitizing this precursor fiber at 3000C for 5.4s (K= 290 WmK ), fiber production can be increased by a factor of approximately six compared to graphitize- tion at 2700C(K=266Wm K ) and by a factor of 670 times compared to graphitization at 2400C (K=270 W The relative energy costs to produce (graphitize) an -o- Fiber A arbitrary unit length of fiber under these different con- -- Fiber B ditions can be estimated from the experimental data given -o- Fiber C in Table 2. The product of the furnace voltage and current and the furnace residence time VX10'XI(s)/3600 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 S/h, is the electrical energy consumption(in kwh) required to process a 3.5 inch (-0.09 m, the furnace hot zone) length of fiber in this laboratory furnace. The calculations 甲中 4. Carbon fiber density as a function of the degree of hitization for fibers heat treated at 2700 C for 2400, 2700, and 3000C may be used to estimate the relative energy costs for fiber production, ignoring other operating costs, such as cooling water and furnace mainte nance. This simple analysis suggests, perhaps contrary to expectations, that the best approach to reduce production costs does not lie in furnace operation at lower tempera greatly reduced residence times and higher throughput 7.00 S@ 00 We have investigated the effects of graphitization time and temperature on the properties of carbon fibers prepared from different mesophase pitch precursors. By using a 0500600.70 continuous mode processing paradigm, it was possible to investigate the effects of very short(sI s)residence times at elevated temperatures on the properties of the carbon Fig. 5. Electrical resistivity as a function of the degree of fibers. Significant variations in density, electrical resistin graphitization for fibers heat treated at 2700C ity, and degree of graphitization were all observed, as were residual effects of precursor type on each of these prop- from the furnace volt and amp requirements. Since fibers erties. The rapid development of desirable material prop- with identical thermal conductivities were not erties was consistent with the known thermal activation nilar conductivities( that ranged fro energy associated with graphitization [9]. Irrespective of 290Wm were used. The residence precursor type, significant densification and decreases in temperatures required to produce carbon fibers with these electrical resistivity were observed for all fibers at a properties(from precursor A)are shown in Table 2. The residence time of only 0.7 s. Corresponding thermal Table 2 290 Wm-i wergy requirements for the manufacture of carbon fibers from precursor fiber A with thermal conductivities between 266 and Heat treatment umace power Residence gy requirements urrent/voltage for production of 0.09 m of fiber (kwh) 165017.5 12.38 1900/9.5 3000 2150/11.1 5.3
M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 1225 different residence times reported imply that different production rates can be expected for the manufacture of carbon fibers with similar properties. For example, by graphitizing this precursor fiber at 3000 8C for 5.4 s 21 21 (k 5 290 W m K ), fiber production can be increased by a factor of approximately six compared to graphitiza- 21 21 tion at 2700 8C (k 5 266 W m K ), and by a factor of 670 times compared to graphitization at 2400 8C (k 5 270 21 21 W m K ). The relative energy costs to produce (graphitize) an arbitrary unit length of fiber under these different conditions can be estimated from the experimental data given in Table 2. The product of the furnace voltage and current 23 and the furnace residence time, VI 3 10 3 t (s)/3600 s/h, is the electrical energy consumption (in kWh) required to process a 3.5 inch (|0.09 m; the furnace hot zone) length of fiber in this laboratory furnace. The calculations Fig. 4. Carbon fiber density as a function of the degree of for 2400, 2700, and 3000 8C may be used to estimate the graphitization for fibers heat treated at 2700 8C. relative energy costs for fiber production, ignoring other operating costs, such as cooling water and furnace maintenance. This simple analysis suggests, perhaps contrary to expectations, that the best approach to reduce production costs does not lie in furnace operation at lower temperature, but furnace operation at higher temperature with greatly reduced residence times and higher throughput rates. 4. Conclusions We have investigated the effects of graphitization time and temperature on the properties of carbon fibers prepared from different mesophase pitch precursors. By using a continuous mode processing paradigm, it was possible to investigate the effects of very short (#1 s) residence times at elevated temperatures on the properties of the carbon Fig. 5. Electrical resistivity as a function of the degree of fibers. Significant variations in density, electrical resistivgraphitization for fibers heat treated at 2700 8C. ity, and degree of graphitization were all observed, as were residual effects of precursor type on each of these propfrom the furnace volt and amp requirements. Since fibers erties. The rapid development of desirable material propwith identical thermal conductivities were not available, erties was consistent with the known thermal activation fibers with similar conductivities (that ranged from 266 to energy associated with graphitization [9]. Irrespective of 21 21 290 W m K ) were used. The residence times and precursor type, significant densification and decreases in temperatures required to produce carbon fibers with these electrical resistivity were observed for all fibers at a properties (from precursor A) are shown in Table 2. The residence time of only 0.7 s. Corresponding thermal Table 2 Analysis of energy requirements for the manufacture of carbon fibers from precursor fiber A with thermal conductivities between 266 and 21 21 290 W m K Heat treatment Furnace power Residence Thermal Energy requirements temp. current/voltage time conductivity for production of 21 21 (8C) (A/V) (s) (W m K ) 0.09 m of fiber (kWh) 2400 1650/7.5 3600 270 12.38 2700 1900/9.5 33.1 266 0.17 3000 2150/11.1 5.3 290 0.04
M L. Greene et al. / Carbon 40 (2002)1217-1226 conductivities for these fibers were as high as 3] Edie DD. Effect of processing on the structure and properties mK. These results indicate that fibers with of carbon fibers. Carbon 1998: 36(4):345-62 performance characteristics can be produced [ Hamada T, Nishida T, Furuyama M, Tomioka T. Transverse short graphitization treatments and, thus, at high structure of pitch fiber from coal tar mesophase pitch Carbon198826(6):837-41 A simple analysis was carried out to estimate the energy [5] Matsumoto T. Mesophase pitch and its carbon fibers.Pure equirements of graphitization for the production of carbon ppl Chem1985:57(11):1553-62. [6] Edie DD, Fox NK, Barnett BC, Fain CC. Melt spun non- fibers with similar thermal conductivities. Contrary circular carbon fibers. Carbon 1986, 24(4 ) 477-82 expectations, the manufacture of these fibers at higl [7 Richardson JH, Zehms EH. Aerospace Corp, El Segundo, temperature for very short times appeared significantly less ech. Report TDR-269(4240-10)-3, 1963(as cited in nergy intensive(350 times less) than graphitization at lower [8 B. In: Proceedings of the 2nd Conference on In- Carbon and Graphite, Soc. Chem. Industry, 1965, p Acknowledgements tion. In: Walker Jr. PL, ed arbon, vol. 7, New York: 可二离 The authors would like to acknowledge bp amoco for in Ref. [ 12] providing the precursor fibers and for financial support of [10) Greene ML. The effects of graphitization time and tempera this research. This research was also partially supported by ture on the electrical resistivity and structural properties of the National Science Foundation under award number cophase pitch-based carbon fibers. MS thesis, Clemso EEC-9731680. We are indebted to Dr. RJ Diefendorf and lemon, SC, 2000. Dr. CC. Fain for enlightening technical discussion [l Foote PD, Fairchild CO, Harrison TR. Pyrometric practice. Technological Papers of the Bureau of Standards, No. 170 References [12] Pacault A. The kinetics of graphitization. In: Walker Jr. PL, ditor, Chemistry and physics of carbon, vol. 7, New York [1] Buckley JD. Carbon-carbon overview [13 Lavin JG, Boyington DR, Lahjani J, Nyusten B, Issi JP DD. editors. Carbon-carbon materia Ridge, NJ: Noyes, 1993, pp. 1-17. Correlation of thermal conductivity with electrical resistivity in mesophase pitch-based carbon fiber. Carbon 2 Edie DD Carbon fiber manufacturing. In: Buckley JD, Edie 1993:31(6:1001-2 DD, editors, Carbon-carbon materials and composites, Park Ridge, N: Noyes, 1993, pp. 319-39
1226 M.L. Greene et al. / Carbon 40 (2002) 1217 –1226 conductivities for these fibers were as high as |250 W [3] Edie DD. Effect of processing on the structure and properties 21 21 m K . These results indicate that fibers with desirable of carbon fibers. Carbon 1998;36(4):345–62. [4] Hamada T, Nishida T, Furuyama M, Tomioka T. Transverse performance characteristics can be produced with very structure of pitch fiber from coal tar mesophase pitch. short graphitization treatments and, thus, at high through- Carbon 1988;26(6):837–41. put rates. [5] Matsumoto T. Mesophase pitch and its carbon fibers. Pure A simple analysis was carried out to estimate the energy Appl Chem 1985;57(11):1553–62. requirements of graphitization for the production of carbon [6] Edie DD, Fox NK, Barnett BC, Fain CC. Melt spun non- fibers with similar thermal conductivities. Contrary to circular carbon fibers. Carbon 1986;24(4):477–82. expectations, the manufacture of these fibers at high [7] Richardson JH, Zehms EH. Aerospace Corp., El Segundo, temperature for very short times appeared significantly less CA, Tech. Report TDR-269 (4240-10)-3, 1963 (as cited in energy intensive (|350 times less) than graphitization at Ref. [9]). [8] Pandic B. In: Proceedings of the 2nd Conference on In- lower temperatures. dustrial Carbon and Graphite, Soc. Chem. Industry, 1965, p. 439. [9] Fischbach DB. The kinetics and mechanism of graphitiza- Acknowledgements tion. In: Walker Jr. PL, editor, Chemistry and physics of carbon, vol. 7, New York: Dekker, 1971, pp. 1–106, as cited The authors would like to acknowledge BP Amoco for in Ref. [12]. providing the precursor fibers and for financial support of [10] Greene ML. The effects of graphitization time and temperathis research. This research was also partially supported by ture on the electrical resistivity and structural properties of the National Science Foundation under award number mesophase pitch-based carbon fibers. MS thesis, Clemson EEC-9731680. We are indebted to Dr. R.J. Diefendorf and University, Clemson, SC, 2000. Dr. C.C. Fain for enlightening technical discussions. [11] Foote PD, Fairchild CO, Harrison TR. Pyrometric practice. Technological Papers of the Bureau of Standards, No. 170, 1921:326. [12] Pacault A. The kinetics of graphitization. In: Walker Jr. PL, References editor, Chemistry and physics of carbon, vol. 7, New York: Dekker, 1971, pp. 107–54. [1] Buckley JD. Carbon–carbon overview. In: Buckley JD, Edie [13] Lavin JG, Boyington DR, Lahjani J, Nyusten B, Issi JP. DD, editors, Carbon–carbon materials and composites, Park Correlation of thermal conductivity with electrical resistivity Ridge, NJ: Noyes, 1993, pp. 1–17. in mesophase pitch-based carbon fiber. Carbon [2] Edie DD. Carbon fiber manufacturing. In: Buckley JD, Edie 1993;31(6):1001–2. DD, editors, Carbon–carbon materials and composites, Park Ridge, NJ: Noyes, 1993, pp. 319–39